1. Field of the Invention
This invention relates to x-ray imaging systems. In a primary application the invention relates to obtaining isolated images of an administered contrast agent.
2. Description of Prior Art
There has recently been a great interest in obtaining images of blood vessels using a noninvasive administration of an iodinated contrast agent. This avoids the dangerous, painful and expensive procedure of inserting catheters into arteries using a surgical procedure. The most common present method of accomplishing this is known as digital radiography. In this approach fluoroscopic television images are taken both before and after the administration of a contrast agent, digitally stored, and subtracted to provide an image of the iodine only. The problem with this system is clearly motion. Any motion between the two stored images will result in severe artifacts in the subtracted image and distort or obliterate the desired image of the iodinated vessel. Even if the patient holds still, he is subject to many involuntary motions of soft tissue structures such as swallowing, respiratory motion, cardiac motions and peristalsis. A describtion of the system is given in the paper by T. Ovitt, et al., "Development of a Digital Video Subtraction System for Intravenous Angiography," Proceedings of the SPIE Conference on Recent and Future Developments in Medical Imaging II, Vol. 205, August 1979, pp. 73-76.
The motion problem can be eliminated by the system described in U.S. Pat. No. 3,848,130 issued to A. Macovski. In this patent, images of various materials are made by making measurements at different regions of the x-ray energy spectrum. These measurements are processed to obtain the desired material images. In this way, iodine can be imaged after administration, without requiring temporal subtraction. In many cases, however, it is difficult or inconvenient to make all of the desired spectral measurements. For example, the separation of iodine from both bone and soft tissue could require measurement at three different energy spectra. The required energy switching can be difficult to accomplish. Also, it is often difficult to efficiently generate all of the desired energy spectra. For example, if a very low energy spectrum is required, below the iodine k edge, the x-ray tube exhibits very low efficiency. It has been found convenient, therefore, to use this selective material imaging system with fewer spectral measurements. If two spectral measurements are made, a more limited set of materials can be separated. For example, two measurements at the lower and upper regions of the diagnostic x-ray spectrum can be processed as described in 3,843,130, to provide an image of iodine and bone components, with the soft tissue cancelled. Similarly, these same two measurements can be combined to obtain an image of iodine and soft tissue, with the bone cancelled. A system of this type is described in a paper by R. E. Alvarez, et al., "Energy Information in X-ray Imaging," Proceedings of the SPSE Conference on Image Analysis Techniques and Applications, January 1981, pp. 150-154. This system, using two spectral measurements, does not, however, provide an isolated image of iodinated blood vessels, free of intervening tissue.
A preferred processing system is described in U.S. Pat. No. 4,029,963, issued to R. E. Alvarez and A. Macovski. Here the two spectral measurements are subjected to a nonlinear processing system to provide two energy-independent components; the Compton scattering component and the photoelectric component. These represent primarily the density and atomic number of each material respectively. Using these two processed data sets, any material can be cancelled by using a linear weighted sum of the two components. This process is described in the previously referenced paper by R. E. Alvarez.